Client 10.4: Settings > Mail > Templates and Signatures > Signatures. The drop-down boxes show a signature selected for new mail and for forwards, none for replies. I select the same signature from the drop down for replies, then click save and close. On replying to a message, no siganture appears, and going back to the settings panel shows the same situation that I thought I had changed, with no reply signature selected.
To be honest, even if I could turn this on I’m not sure I would until there is a distinction between the first reply and subsequent ones, but I guess that’s a seperate feature request…
Apart from setting the signature for replies in the field, it could be due to eg: the signature position field option for either “before quoted text” or “after quoted text” as in the below thread.